Method of making accurate cores

ABSTRACT

Method for making shell-type cores which involves joining together a plurality of core patterns which when joined provide a hollow composite pattern, each of the patterns having ribs extending therealong. The composite pattern is invested in a ceramic mold making material to form outer and inner ceramic shells about the composite pattern. A portion of the outer shell is then removed to expose at least a portion of the ribs. Upon melting out of the pattern, the discontinuity created by the presence of the ribs causes the outer shell to come apart and leaves the inner shell intact. The inner shell which is in the configuration of the desired core is then fired to provide a rigid shell-type core.

United States Patent July 11,1972

Blazek [54] METHOD OF MAKING ACCURATE CORES [72] Inventor: William S.Blazek, Alliance, Ohio [73] Assignee: TRW Inc., Cleveland, Ohio [22]Filed: Dec. 16, 1970 [21] Appl. No.: 98,571

[52] US. Cl... ...l64/35, 164/246, 164/247 [51] Int. Cl. ..B22c 9/02[58] Field otSearch ..l64/34, 35, 36

[56] References Cited UNITED STATES PATENTS 3,424,227 l/l969 Watts et a]..l64/36 Primary Examiner-J. Spencer Overholser Assistant Examiner-V. K.Rising Anomey-Hill, Sherman, Meroni, Gross & Simpson [57] ABSTRACTMethod for making shell-type cores which involves joining together aplurality of core patterns which when joined provide a hollow compositepattern, each of the patterns having ribs extending therealong. Thecomposite pattern is invested in a ceramic mold making material to formouter and inner ceramic shells about the composite pattern. A portion ofthe outer shell is then removed to expose at least a portion of theribs. Upon melting out of the pattern, the discontinuity created by thepresence of the ribs causes the outer shell to come apart and leaves theinner shell intact. The inner shell which is in the configuration of thedesired core is then fired to provide a rigid shell-type core.

6Clains,5DrawingFigures PKTENIEDJULH I972 v 3,675,708

INVENTOR.

BY M M, W Women METHOD OF MAKING ACCURATE CORES BACKGROUND OF THEINVENTION I. Field of the Invention This invention is in the field ofmaking ceramic type cores for use in precision investment castingprocesses and involves forming inner and outer shells about a compositepattern, simultaneously removing the composite pattern and the outershell, and then firing the remaining inner shell to produce a rigidshell-type core.

2. Description of the Prior Art In the manufacture of complex castingshaving passages or voids therein which must meet close tolerances, it iscommon practice to employ preformed ceramic cores in the investmentmold. The problem of securing cores with suitable accuracy anddimensions and with the smooth surface finish required poses a problemparticularly in the case of large cores.

Preformed ceramic cores are usually made either from fused silica tubingor by pressing or molding powdered or granular refractory material intothe required shape and firing it. In the case of molding the preformedcore, the mold used must be as dimensionally accurate as the tooling forproducing the patterns, and should be hardened to resist the abrasivewear caused by the injection of ceramic material. All of these factorsadd considerably to the cost of the overall operation.

SUMMARY OF THE INVENTION In accordance with the present invention, aceramic core is produced by first providing a composite pattern made upof two or more disposable pattern sections. These sections are joined bymeans of heat or solvent welding, and each section is provided with arib running lengthwise of the section. The ribs on the joined sectionsmerge to form a continuous rib running the length of the compositepattern.

The composite pattern is then dipped into suitable ceramic slurries tobuild up thin ceramic shells on the interior and the exterior of thepattern surfaces. After drying, a portion of the exterior shell thusproduced is removed in the vicinity of the rib, thereby exposing atleast a portion of the rib along the length of the shells. The patternmaterial is then melted out, causing the outer shell to come apart andleaving the inner shell which, upon firing, becomes a rigid ceramic coresuitable for use as a preformed core in subsequent mold makingprocesses.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantagesof the invention will be readily apparent from the following descriptionof certain preferred embodiments thereof, taken in conjunction with theaccompanying drawings, although variations and modifications may beeffected without departing from the spirit and scope of the novelconcepts of the disclosure, and in which:

FIG. 1 is a view in elevation illustrating two wax patterns which arecombined to form the composite pattern;

FIG. 2 is a cross-sectional view taken substantially along the line IIIIOF FIG. 1;

FIG. 3 is a cross-sectional view of the composite pattern after it hasbeen invested with a ceramic mold making material so as to form innerand outer ceramic shells about the pattern;

FIG. 4 is a crosssectional view taken along the line IV-IV of FIG. 3,after a portion of the outer shell has been removed to expose the ribsin the composite pattern; and

FIG. 5 is an end view of the completed core, showing the manner in whichthe outer shell is broken away during melting of the pattern material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, there is illustrateda pair of pattern sections and 11 which, when combined, form a compositepattern having the shape desired in the finished core of the presentinvention. As illustrated, pattern section 10 has a relatively smalldiameter end portion 12 which forms a core print for one end of thecore, and the pattern section 11 has a relatively larger diameter endportion 13 forming a core print for that end of the core.

The pattern section 10 has a pair of ribs 14 and IS on its peripheryextending longitudinally along the surface thereof. Similarly, thepattern section 11 has a pair of ribs 16 and 17 also extending along thelongitudinal dimension. When the two pattern sections 10 and 11 are puttogether, the ribs 14 and 16, as well as the ribs 15 and 17 abut to forma continuous rib on both sides of the resulting composite pattern. Thetwo pattern sections 10 and 11 may be joined together by heat in thecase of wax, or they may be solvent welded in the event a plasticpattern material such as polystyrene is employed.

The resulting composite pattern is then dipped successively into ceramicslurry compositions, with intermediate drying, in order to build up anouter shell 18 surrounding the outer periphery of the pattern and theribs, and an inner shell 19 interiorly of the hollow composite pattern.The ceramic slurries which are used to build up such shells are wellknown in the art and do not fonn a novel feature of the presentinvention. By way of illustration, however, such slurries may containfused silica, zircon, or other refractory materials in combination withbinders. Chemical binders such as ethyl silicate, sodium silicate andcolloidal silica have been used very extensively. The coating materialmay also contain other additives to perform specific functions. Forexample, the slurry may contain a film former such as an alginate tocontrol viscosity. A wetting agent may be included to control flowcharacteristics and pattern wettability. An anti-foam agent such as asilicone emulsion may be used to remove entrapped air. An anti-dryingagent such as gelatin can be added to control drying and reduceshrinkage cracks.

The initial coating applied to the pattern contains very finely dividedrefractory material, to produce an accurate surface finish. A typicalslurry for a first coat might contain 29 percent of a colloidal silicasuspension in the form of a 20 to 30 percent concentrate. Fused silicaof a particle size of 325 mesh or smaller in an amount of7l percent canbe employed, together with less than one-tenth percent by weight of awetting agent. Generally, the specific gravity of such a slurry is onthe order of 1.75 to 1.80 and has a viscosity of 40 to 60 seconds whenmeasured with a Number 5 Zahn cup at 75 to F.

After application of the initial coating, the surface is stuccoed withrefractory materials having particle sizes on the order of 60 to 200mesh.

Each coat is dried before subsequent dipping in succeeding ceramicslurries. Generally, from three to eight dips are employed to build up athickness of one-eighth to one-half inch.

When the outer or inner shells 18 and 19 have been built up to asuitable thickness, and dried, the shells are self-sustaining but notrigid. At this stage, the outer shell 18 is ground or otherwise abradedto expose at least a portion of the ribs as shown in FIG. 4 of thedrawings. The grinding away of the shell material about the ribs thusleaves the outer shell discontinuous along the parting lines provided bythe composite ribs 14-16 and 15-17.

The next step consists in removing the pattern material, typically byfiring it in a furnace or by treating it in an autoclave. When thepattern material liquefies, the liquid material flows from between thetwo shells, so that the outer shell separates into two sections and 18b,as illustrated in FIG. 5, leaving the inner shell 19. The outer surfaceof the shell 19 is very smooth and requires no additional surfacetreatment. The shell is formed into a finished ceramic core by firingthe same at firing temperatures which usually range from about l,500 tol,900 F. The rigid ceramic shell thus produced is then usable in othermold making processes, such as by injecting additional amounts ofpattern material about it to eventually form the finished moldstructure.

With the type of preformed core employed in the present invention, it isconvenient to make the core with physical properties different from theremainder of the refractory shell in which it is ultimately included.For example, it can be made lOlO-M 03H stronger, more leachable or havedifferent dimensional expansion characteristics from the rest of therefractory shell. The biggest advantage is the obtainment of anexcellent surface finish and dimensional accuracy in an inexpensiveprocess. Another advantage resides in the fact that the preformed coreis easily inspected and repaired if defects are found. The size rangefor the core is unlimited, and it can be easily handled. The process ofmaking the core is considerably less expensive than the moreconventional methods now used for the manufacture of preformed ceramiccores.

I claim as my invention:

1. The method of making a shell-type core which comprises joiningtogether a plurality of core patterns which when joined provide a hollowcomposite pattern, said patterns each having ribs extending from theperipheries thereof, investing the composite pattern in a ceramic moldmaking material to form outer and inner ceramic shells about saidcomposite pattern,

removing a portion of the outer shell to thereby expose at least aportion of said ribs, melting out the pattern to thereby cause ofpolystyrene.

6. The method of claim 1 in which said core patterns are joined togetherby heat.

I! t I t

1. The method of making a shell-type core which comprises joiningtogether a plurality of core patterns which when joined provide a hollowcomposite pattern, said patterns each having ribs extending from theperipheries thereof, investing the composite pattern in a ceramic moldmaking material to form outer and inner ceramic shells about saidcomposite pattern, removing a portion of the outer shell to therebyexpose at least a portion of said ribs, melting out the pattern tothereby cause the outer shell to come apart and leaving the inner shellintact, and firing the inner shell to provide a rigid shell-type core.2. The method of claim 1 in which said ribs extend longitudinally ofsaid patterns.
 3. The method of claim 1 in which said ribs arepositioned to provide a continuous rib extending the length of thecomposite pattern.
 4. The method of claim 1 in which said pattern iscomposed of wax.
 5. The method of claim 1 in which said pattern iscomposed of polystyrene.
 6. The method of claim 1 in which said corepatterns are joined together by heat.